decrementable uses hidden friend

Author: rollbear

include/strong_type/strong_type.hpp

@@ -603,22 +603,23 @@ struct decrementable
   class modifier
   {
   public:
+    friend
     STRONG_CONSTEXPR
     T&
-    operator--()
+    operator--(T& t)
     noexcept(noexcept(--std::declval<T&>().value_of()))
     {
-      auto &self = static_cast<T&>(*this);
-      --value_of(self);
-      return self;
+      --value_of(t);
+      return t;
     }
 
+    friend
     STRONG_CONSTEXPR
     T
-    operator--(int)
+    operator--(T& t, int)
     {
-      auto copy = static_cast<T&>(*this);
-      --*this;
+      auto copy = t;
+      --t;
       return copy;
     }
   };